Page:Highway Needs of the National Defense.pdf/121

Rh Added lanes

One means available to the highway designer for elimination or lessening of the congestion that is created by heavy vehicles crawling up hills is the construction of added lanes or truck lanes on the uphill side of the grades. This method has been employed by several States with excellent results.

When the traffic volume on a two-lane road does not exceed 300 vehicles per hour the congestion on grades resulting from slow-moving vehicles in numbers normally found on main highways is not sufficient to justify the construction of a third lane, regardless of the alinement and profile, for grades up to 7 percent.



Where the alinement and profile are such that sight distance does not restrict the performance of passing maneuvers, added lanes are not justified for hourly volumes below 500 vehicles. But this is seldom the case, because sight distance is commonly restricted at the crests of hills. With the sight-distance restrictions that are normally present on sections that include grades, an added truck lane is justified under the conditions shown in table 12.

Where the traffic volumes approach those that require a four-lane divided highway, the added lane will provide temporary relief only and, under these conditions, in some instances at least, the construction of a divided four-lane highway is the more feasible solution, especially if the level sections of the highway have a poor alinement.